This invention relates to an image forming process and, more specifically, to a full color image forming process.
In electrophotography and electrostatic recording or printing, there are known a wet developing method in which a liquid developer is used for developing a latent image and a dry developing method in which a powder developer is used for developing a latent image. Because the powder developer generally has a coarse particle size of as high as about 7 xcexcm, the dry developing method has a limitation in obtaining very fine images. On the other hand, the liquid developer uses developing particles with a diameter of 2 xcexcm or less, generally 1 xcexcm or less, fine, high quality images can be obtained. Because of this advantage, an attention is now again being paid upon the wet developing method. Namely, the developer for use in the wet developing method is composed of a color fine powder (toner particles) dispersed in an electrically insulating organic solvent. Since the particle size of the toner particle is very fine, the reproducibility of fine line images is much improved as compared with that attained by the dry developing method. Such an advantage as well as high resolution, contrast and gradient are also obtainable in the development of a full color images obtained by using yellow, magenta, cyan and black toners.
With a view toward improving copying speed while retaining the above advantages of the liquid developer, U.S. Pat. No. 4,945,387 and U.S. Pat. No. 4,984,025 suggest an image forming method using an intermediate image transfer medium. The use of the intermediate image transfer medium has an additional merit that toner images can be transferred not only a sheet like transfer medium such as paper but also other transfer media. The known methods adopting an electrostatic transfer system, however, still has a limitation in obtaining high quality copies at a high speed.
U.S. Pat. No. 4,708,406 proposes a method in which a toner image on an image supporting roller is transferred to an intermediate transfer medium in the form of a belt or a roller by pressure contact therebetween. In this method, however, the transfer efficiency is not satisfactory so that the quality of the copy image is not fully satisfactory.
International Publication No. WO90/05942 discloses a method in which a toner image on an intermediate image transfer medium is heated to impart adhesiveness thereto and to improve the transfer efficiency and fixation efficiency. This method, however, causes deterioration of the toner image on the intermediate image transfer medium due to heat applied thereto, so that reliable images are not obtainable.
In a wet developing method, it is known that a sharp image having good resolution may be obtained by previously charging a toner image on a photosensitive image supporting member prior to the image transfer. Such an improvement in sharpness is considered to be attained by the following mechanism.
Referring to FIG. 4, designated as 1 is a photosensitive image supporting member having a developing liquid layer 2 in which an image of toner particles 3 has been just developed. Each toner particle is composed of a pigment 5 and a resin layer 6 surrounding the pigment 5. The toner image is not tightly bound to the supporting member 1 because the toner particles 3 are in a swelled state. As a consequence, when the toner image-bearing supporting member 1 is brought into pressure contact with a transfer paper, the toner image is deformed so that a sharp image is not obtainable.
When the toner image-bearing supporting member is charged prior to the image transfer, the toner particles 3 are tightly bound to the supporting member 1 as shown in FIG. 5. As a consequence, the toner image is able to be transferred to a transfer paper without being disturbed or spread, so that a sharp image is obtainable.
It has been found, however, that the above previous charging system gives a high quality image only when a high quality transfer paper having a smoothness of 100 seconds or more is used. In particular, when a transfer paper having a smoothness of 60 seconds or less is used, a uniform solid pattern is not obtainable. Namely, the concentration (density) of the solid pattern is not uniform. Moreover, image density and sharpness are not satisfactory when a low grade transfer paper is used.
It is, therefore, an object of the present invention to provide an image forming process which can give uniform solid pattern on a transfer medium even when the surface smoothness of the transfer paper is not good.
Another object of the present invention is to provide an image forming process of the above-mentioned type which can give images having excellent sharpness.
It is a further object of the present invention to provide an image forming process of the above-mentioned type which can give images having a high density and high resolution.
In accomplishing the foregoing objects, there is provided in accordance with the present invention an image forming process comprising the steps of:
(a) contacting a latent image-bearing surface of a supporting member with a toner-containing liquid developer to develop said latent image with said liquid developer and to form a toner image covered with a layer of said liquid developer;
(b) treating said toner image-bearing surface with a squeezing member to squeeze said liquid developer layer therefrom;
(c) then treating said toner image-bearing surface with a voltage impressing member to impart a bias voltage to said toner image;
(d) then transferring said toner image from said surface to an intermediate transfer medium; and
(e) then transferring said transferred toner image from said intermediate transfer medium to a transfer medium.